[Hexagon] Handle truncates in polynomial multiply idiom recognition

This is in anticipation of https://reviews.llvm.org/D42424, which would
otherwise break one of the pmpy testcases.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323824 91177308-0d34-0410-b5e6-96231b3b80d8